As known, in the provision of non-volatile memory devices, e.g., of the EPROM and flash EPROM type, there are used certain techniques to obviate the low yield of the production process.
These techniques consist of providing a non-volatile memory cells matrix with additional rows or columns--termed redundant--which can be used to replace defective rows or columns which display malfunctions subsequent to testing of the memory device.
The function of the redundant elements is basic for the reliability of the memory device.
Those skilled in the art know the design methodology and the use of redundant rows and columns, of trimming networks and propagation control and of UPROM cells and the related selection circuitry. The latter allows readdressing the memory in such a manner as to replace the addresses containing defective bits with those containing functioning bits present in the redundant rows or columns.
For example, a UPROM cell is programmed in the testing phase, i.e., at the time the memory devices are subjected to an EWS (Electrical Wafer Sort). Before performing any kind of operation on the memory device the cells of the UPROM circuitry which will permit correctly addressing the memory addresses to be replaced are read.
The redundant elements are necessarily of the non-volatile type and are divided in two large groups, i.e., those modifiable as needed, e.g., EEPROM, and those modifiable only once such as, for example, electrically programmable fuses, PROM cells, etc.
The redundant elements thus allow reconfiguration of a non-volatile memory displaying malfunctions and may be defined as configuration elements. It is clear however that to fulfill their function the configuration elements must be whole and not display defects in turn.
It may happen that such configuration elements undergo accidental modifications due to undesired and unforeseeable events such as, for example, falls or overshoots of the power supply voltage or electrostatic discharges. These phenomena are known as `soft writing` of the configuration elements, for example the UPROM cells.
It is clear that if a configuration element of the modifiable only once type is damaged by an electrostatic discharge the characteristics of the related memory device can even be modified permanently and its functionality is compromised.